Inhibition of the formation and stability of inorganic colloids in the alkaline disturbed zone of a cementitious repository

The generation and stability of inorganic colloids have been studied under hyperalkaline conditions. For the generation of colloids, intact cores of Bromsgrove Sandstone were flushed with simulated cement leachates, and the eluates were ultrafiltered sequentially (12 μm, 1 μm, 0.1 μm and 30 kDa) for the separation of any colloids found. No colloid formation was observed during the experiments; however the analysis by ICP-MS of the eluates showed significant increases in Si and Al, indicating silicate mineral dissolution, as well as reduction of the concentration of Ca in the leachates indicating precipitation of secondary Ca-rich phases. Flow experiments with cement leachates spiked with tritiated water showed a noticeable reduction of the porosity of the sandstone as well as changes in the pore distribution. Additional stability experiments were carried out using model silica and Fe2O3 colloids. The experiments indicated that the stability of the colloids was mainly controlled by the concentration of Ca in solution and that both types were unstable under the chemical conditions in the alkaline disturbed zone. The presence of cement additives such as superplasticisers could enhance the stability of the colloids.